3:2 Pull-down detection

ABSTRACT

In one embodiment, a 3:2 pull-down detection component of a video encoder uses motion vectors to determine whether a repeated field exists in a video sequence. The 3:2 pull-down detection component uses field motion vectors determined by a motion estimator and compares the field motion vectors to a threshold to determine whether a repeated field exists. If a repeated field exists, a video encoder can then eliminate the repeated field.

RELATED APPLICATIONS

[0001] The present application claims the benefit of U.S. provisionalpatent applications Ser. Nos. 60/254,804, 60/254,953 and 60/254,809, allfiled on Dec. 11, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to devices and methods forefficiently encoding digital video.

RELATED ART

[0003] One type of film runs at 24 Hz. That is, twenty four frames offilm are displayed every second. In the United States, according to theNational Television System Committee (NTSC) standards, television videoruns at 30 Hz. When converting film to be shown on television, problemsarise because of the extra frames needed for every second of televisionbroadcast. More specifically, there are six more frames of televisionvideo every second than corresponding film frames, and in order todisplay film on television with proper timing something must be done tofill in the last six frames. Further, according to the NTSC standard,television video is interlaced. That is, every frame is further made upof two fields, a top field and a bottom field. So, for every second, 60fields of video are shown.

[0004] In order to solve the problem of having extra video frames whenconverting film to be shown on television, the 3:2 pull down processconverts two frames of film into five fields of video. One method ofperforming this process involves repeating one of the fields. Morespecifically, this method involves converting the two frames of filminto two frames of video, each frame of video having two fields, andthen repeating one of the video fields to correct the timing.

[0005] Digitally encoded video is typically compressed because video canrequire an enormous amount of digital storage if left uncompressed. Onemethod for compressing digital video involves using the standards of theMoving Pictures Experts Group (MPEG). The MPEG-2 standard calls forthree types of frames to be encoded. Intra-frames, or I-frames areencoded in the same manner as still images; an I-frame containsinformation sufficient to display an entire image. Predictive frames, orP-frames use previous reference frames to determine what the currentframe will be by recording changes between a previous frame and thecurrent frame. Bi-directional frames, or B-frames use previous andsubsequent reference frames to determine what the current frame will be.P-frames and B-frames use motion vectors to encode frames.

[0006] A motion vector determines movement between specific areas of oneframe to another frame. For example, a P-frame may be encoded byreferencing an I-frame immediately preceding it. Motion vectors betweenthe P-frame and the I-frame instruct a decoder to display the P-frame byusing motion vectors to determine movement of certain areas within theI-frame which results in the proper display of the P-frame.

[0007] More specifically, each frame can be divided up into a number ofmacroblocks. A macroblock is a group of pixels; for example a macroblockcould be a square 16 pixels by 16 pixels. A motion vector can thenrecord the movement of a macroblock in a first frame to its new positionin a second frame. For example, a macroblock in a first frame could be ablack 16 by 16 pixel square in the lower left hand corner. In the secondframe, the black square may move to the upper right hand corner of theframe. Instead of recording the characteristics of the black square inthe second frame, the second frame can instead have a motion vectorindicating that the black square, which was in the lower left handcorner in the first frame, has moved to the upper right hand corner inthe second frame. Since a macroblock will generally contain much moredata information than a motion vector which indicates the direction ofmovement of a previously encoded macroblock, motion vectors can greatlyreduce the amount of data necessary for digital video.

[0008] One method of encoding digital video calls for grouping framestogether into what are known as Groups of Pictures (GOPs). A GOP maybegin with an I-frame, and have P-frames and B-frames which refer to theI frame. A P-frame or a B-frame can refer to either an I-frame or aP-frame, but not to a B-frame. The length and order of GOPs can bedetermined before encoding or dynamically, while the encoder isencoding. An example of a sequence of a GOP may be IBBPBBPBBI, meaningan I-frame, followed by two B frames, a P frame, two more B-frames,another P-frame, two more B-frames, and an I-frame. In an encoder whichdetermines the order of a GOP prior to encoding, this sequence wouldrepeat itself. In the above sequence, the first P-frame will refer backto the first I-frame, since it cannot refer to a B-frame, and must referto a frame that occurs before it. The B-frames may refer to any of theI- or P-frames.

[0009] One method of applying 3:2 pull down introduces a repeated fieldfor every five fields of video. When encoding video, which at one timewas film, the 3:2 pull down process leaves a repeated field as one outof every five fields. This repeated field can be detected and removed.By removing repeated fields, the encoding process can be made moreefficient, and ultimately the amount of the resulting data can begreatly reduced. However, current methods for detecting repeated fields,such as pixel to pixel matching from field to field, can require toomuch processing time and too many resources. Therefore, an efficient andeffective method for determining which fields are repeated is needed.

[0010] Further, it is advantageous for the encoder to be able to detectwhen a new scene is beginning in a video sequence. Current methods fordetecting a scene change include histogram-based algorithms and blockmatching algorithms. These methods are very processor intensive, andgenerally cannot be used for real-time digital video encoding.Therefore, an efficient and effective method for detecting scene changesduring digital video encoding is needed.

SUMMARY OF THE INVENTION

[0011] In one embodiment, a 3:2 pull-down detection component of a videoencoder uses motion vectors to determine whether a repeated field existsin a video sequence. The 3:2 pull-down detection component uses fieldmotion vectors determined by a motion estimator and compares the fieldmotion vectors to a threshold to determine whether a repeated fieldexists. If a repeated field exists, a video encoder can then eliminatethe repeated field.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a flow diagram illustrating the process of a videoencoder according to one embodiment.

[0013]FIG. 2 illustrates a system for encoding and decoding digitalvideo according to one embodiment.

[0014]FIG. 3a illustrates a video encoder and associated hardware,according to one embodiment.

[0015]FIG. 3b illustrates an encoder according to one embodiment.

[0016]FIG. 4 is a timing diagram for a video sequence without scenechange or repeated fields.

[0017]FIG. 5 is a timing diagram for a video sequence with repeatedfields.

[0018]FIG. 6 is a timing diagram for a video sequence with an I-frame inbetween two repeated fields.

[0019]FIG. 7 is a timing diagram for a video sequence with a scenechange during a top-field-first situation.

[0020]FIG. 8 is a timing diagram for a video sequence with a scenechange during a bottom-field-first situation.

[0021]FIG. 9 is a flow diagram illustrating the operation of an encoderaccording to one embodiment.

[0022]FIG. 10 illustrates a video sequence having a scene at a B-frame(immediate) right after an I-frame.

[0023]FIG. 11 illustrates a video sequence having a scene changeoccurring two frames after an I-frame.

[0024]FIG. 12 illustrates a video sequence having a scene changehappening at a B-frame.

[0025]FIG. 13 illustrates a video sequence having a scene changeoccurring at a P-frame.

[0026]FIG. 14 illustrates a video sequence having a repeated field in anI-frame.

[0027]FIG. 15 illustrates a video sequence having a repeated field in aB-frame.

[0028]FIG. 16 illustrates a video sequence having a repeated field in aP-frame.

[0029]FIGS. 17a, 17 b, 17 c are block diagrams of an encoder accordingto one embodiment.

[0030]FIG. 18a illustrates two video frames and their associated motionvectors according to one embodiment.

[0031]FIG. 18b illustrates a sequence of frames operating in atop-field-first condition.

[0032]FIG. 18c illustrates a sequence of frames with a repeated field ina top-field-first condition.

[0033]FIG. 18d illustrates a sequence of frames operating in abottom-field-first condition.

[0034]FIG. 18e illustrates a sequence of frames with a repeated field ina bottom-field-first condition.

[0035]FIGS. 19a, 19 b, 19 c, 19 d, 19 e, and 19 f illustrate a sequenceof frames having repeated fields according to one embodiment.

[0036]FIG. 20 illustrates two frames and their associated motion vectorsaccording to one embodiment.

[0037]FIG. 21 is a flow diagram illustrating the process of detectingrepeated fields according to one embodiment.

[0038]FIGS. 22a, 22 b, and 22 c illustrate a sequence of framescontaining a scene change according to one embodiment.

[0039]FIG. 22d illustrates an interlaced video sequence having a scenechange.

[0040]FIG. 22e illustrates a progressive video sequence having a scenechange.

[0041]FIG. 23 illustrates two frames and their associated motion vectorsaccording to one embodiment.

[0042]FIG. 24 is a flow diagram illustrating the process of detectingrepeated fields according to one embodiment.

DETAILED DESCRIPTION

[0043] The present invention relates to devices and methods forefficiently encoding digital video. This invention may be used toincrease efficiency when encoding video that has been processed using a3:2 pull down process. Although the embodiments described below relateto encoding video that has been processed using a 3:2 pull down process,it is understood that the present invention may be used for any type ofvideo.

[0044]FIG. 1 is a flow diagram illustrating the process of a videoencoder according to one embodiment. The encoder accepts a videosequence as an input, and outputs a digitally encoded video bitstream.According to one embodiment, the video encoder encodes video accordingto an MPEG standard. The process illustrated in FIG. 1 is generallydescribed; more detail will be added in figures following. It isunderstood that while the process of FIG. 1 illustrates one embodimentof the invention, there are numerous methods of encoding video, and oneskilled in the art will realize that the features of FIG. 1 can beintegrated into any number of different encoders.

[0045] At block 105, pre-filtered output is inputted into the videoencoder. The pre-filtered output of block 105 is first sent to phase ofa motion estimator in block 110. A motion estimator, which determinesmotion vectors to encode frames, is here split into two separate phases.Phase one of the motion estimator entails determining two sets of motionvectors, one set between the first field of a first frame and the firstfield of a second frame and another set between a second field of afirst frame and a second field of a second frame. The second phase ofthe motion estimator determines the remaining motion vectors: thosebetween the first field of the first frame and the second field of thesecond frame, those between the second field of the first frame and thefirst field of the second frame, and those between the first frame andthe second frame. The motion vectors indicate motion of macroblocksbetween two different frames. In a standard interlaced video sequence inwhich one top field and one bottom field comprise a frame, the firstphase of the motion estimator determines motion vectors between fieldsof the same polarity. That is, two top fields are said to have the samepolarity, but a top field and a bottom field are said to have oppositepolarity. The motion vectors between fields of the same polarity arealso known as field motion vectors.

[0046] By bifurcating the motion estimation phase, the field motionvectors may be used in the scene change detection and 3:2 pull-downdetection phases in blocks 115 and 120, respectively. The field motionvectors can be used during scene change detection and 3:2 pull-downdetection to generate additional information for these detections suchas histograms and fields difference calculations, rather than using moreprocessor intensive methods. Because it is necessary to determine fieldmotion vectors for each frame in order to encode the frame, using thefield motion vectors to perform the detections may introduce littleextra processing into the system. It may also be advantageous tobifurcate the motion estimation phase because the result of thedetections may render the second phase of the motion estimation phaseunnecessary, and if the second phase is found to be unnecessary, theencoder can forgo estimating of the remaining motion vectors, savingfurther processing resources.

[0047] Previous video encoders had to encode I- and P-frames beforeB-frames were encoded because a B-frame uses I- and P-frames asreferences, and can reference to a frame in the future. However, using atwo phase motion estimation, an encoder can encode frames using theinput sequence of frames.

[0048] At block 115, a scene change detection is executed. Scene changedetection uses the field motion vectors found in block 110 to determinewhether the scene has changed between two frames. Generally, if there isno scene change between frames, one can expect the motion vectors of aframe to be similar to the motion vectors of the frame before it.However, if there is a scene change between frames, the motion vectorswill become unpredictable and erratic. By comparing these motionvectors, it can be determined that there has been a scene change. Oneembodiment of a scene change detection process will be explained in moredetail below.

[0049] If a scene change is detected, the encoder can order thebeginning of a new Group of Pictures (GOP) immediately or soon after thescene change. As explained above, whenever a scene change occurs, thefirst frame of the scene change will have motion vectors which are veryerratic and large in magnitude. As a result, these motion vectors willnot be of much use. It may therefore be advantageous to begin a new GOPwith a new I-frame, so that frames at and after the scene change do nothave to refer to frames in an earlier scene which may be very different.The encoder's response to the detection of a scene change will also beexplained in more detail below.

[0050] If there is no scene change, the process then moves on to block120, where 3:2 pull-down detection is executed. As explained above, the3:2 pull-down process introduces one field out of every five that isrepeated from another field. Because this repeated field is identical toanother field, a processing and data storage savings can be realized byreplacing the repeated field with a reference to the earlier field fromwhich it is repeated.

[0051] The 3:2 pull-down detection process of block 120 involves usingthe field motion vectors determined by the first phase of the motionestimator in block 110 to determine whether there is a repeated field.Because motion vectors indicate the motion of macroblocks, if one fieldis repeated from another, any motion vectors between those two fieldsshould theoretically have a magnitude of zero. In reality, there willalways be some noise in any video system, but if the sum of themagnitudes of one set of field motion vectors is significantly greaterthan the sum of the magnitudes of the other set of field motion vectors,then the fields which are related by the smaller sum of magnitudes canbe said to be repeated. For example, if the top fields in two frames arerepeated, the sum of the magnitudes of the motion vectors relating thetwo top fields will be significantly less than the sum of the magnitudesof the motion vectors relating the bottom fields. This is explained inmore detail below.

[0052] If a repeated field is found, the repeated field can be replacedby a reference to the previous field and encoding begins again bysupplying new frames to the motion estimator. However, if there is norepeated field, the encoding process continues. If there is a scenechange, the 3:2 pull down detection can be bypassed and the processmoves on to picture heading encoding in block 125.

[0053] Once the step of block 125 is completed, the process moves on tomacroblock level encoding in block 130. Macroblock level encoding,including the second phase of the motion estimator and the mode decisionfor the best motion vector for each macroblock, encodes a frame atmacroblock basis. The second phase of the motion estimator includesdetermining the motion vectors between the first field of a first frameand the second field of a second frame, between the second field of afirst frame and the first field of a second frame and between a firstframe and a second frame. Macro-block level encoding at block 130completes the encoding for a specific frame. Once encoding is completed,the next frame may be entered into the encoder and the process beginsagain.

[0054] Significant processing time can be saved by dividing the motionestimator into two discrete phases. The first phase, determining thefirst two sets of motion vectors, can be performed before the scenechange detection, the 3:2 pull-down detection, and the second phase ofthe motion estimator. The results of the first phase can determinewhether there is a scene change. Once a scene change is found it is nolonger necessary to execute the 3:2 pull-down detection and the secondphase of motion estimation. If there is no scene change detected, the3:2 pull-down detection will be executed. If there is a repeated field,it need not be encoded, but a reference to the field it is repeated fromcan be inserted. Thus, the second phase motion estimation for a repeatedfield does not need to be executed. The processing resources saved fromnot encoding repeated fields can be used for encoding other frames toimprove the quality of the video. Therefore, determining the fieldmotion vectors first and using them to find repeated fields and scenechanges can significantly reduce the amount of processing required andimprove quality.

[0055]FIG. 2 illustrates a system for encoding and decoding digitalvideo according to one embodiment. Film at 24 Hz 205 can be processed ina telecine 210 which performs 3:2 pull-down to create 30 Hz video. The30 Hz video is transferred as an analog broadcast to an end user. Adevice, having a video code 215 may then process the 30 Hz analogbroadcast. An encoder 220, takes the 30 Hz video and encodes it at 24 Hzby removing repeated fields as in the processes explained above. Thevideo may then be stored on data source 225. Then, the 24 Hz encodedvideo may be decoded by decoder 230 and returned to 30 Hz video byinserting the repeated fields, which may be viewed by a user.

[0056]FIG. 3a depicts a processing system 300 in which one embodiment ofthe invention may be implemented. For one embodiment, a video encodingdevice may be implemented using a general processing architecture.Referring to FIG. 3a, the system may include a bus 302, or othercommunications means for communicating information, and a centralprocessing unit (CPU) 304 coupled to the bus for processing information.CPU 304 includes a control unit 306, an arithmetic logic unit (ALU) 308and registers 310. CPU 304 can be used to implement the video encoderand decoder. The processing system 300 also includes a main memory 312,which may be a random access memory (RAM) device that is coupled to thebus 302. The main memory stores information and instructions to beexecuted by CPU 304. Main memory 312 may also store temporary variablesand other intermediate information during the execution of instructionsby CPU 304. The system 300 also includes a static memory 314, forexample, a read-only memory (ROM), and/or other static device that iscoupled to the bus 302 for storing static information and instructionsfor CPU 304. It should be realized that processor executableinstructions, reflective of the processes described herein may be storedin one of the memories referred to above and/or stored or transferredthrough some other computer readable medium.

[0057] The encoder 320 is coupled to the bus 302 and configured toencode digital video. The encoder 320 includes a motion estimator 322having a first phase 324 and a second phase 326. The motion estimator isused to determine motion vectors. The first phase 324 of the motionestimator determines the field motion vectors as described above andbelow. The second phase 326 of the motion estimator determines thethird, fourth, and fifth sets of motion vectors as described above andbelow. The encoder 320 further includes a scene change detectioncomponent 328 to detect scene changes between frames, as explainedearlier and further below. Encoder 320 also includes a 3:2 pull downdetection component 330 to detect whether there are repeated fields inany frames, as explained earlier and below. In one embodiment, theencoder operates in a manner as explained in the flow diagram in FIG. 1.

[0058]FIG. 3b illustrates an encoder 350 according to one embodiment.Frames are input at point 352 and saved into frame buffer 354 for motionestimation. The field motion vectors V 370 from phase-one motionestimator 358 are stored in field motion vector buffer 356 for scenechange detector 360 and 3:2 pull-down detector 362 to use. The scenechange detector 360 sends the detection outcome 372, an indicator i_sc,to inform encoder controller 366. The 3:2 pull-down detector 362 sendsthe detection outcome 374, and indicator i_pd, to inform the encodercontroller 366 and to eliminate repeated field.

[0059] According to one embodiment, the flow of the encoder is explainedabove, in FIG. 1. Motion estimation phase 1 358 accepts inputted framesand computes field motion vectors for the inputted frames. As explainedabove, the phase 1 358 only computes the field motion vectors, which arethe motion vectors between fields having the same polarity. Scene changedetection component 360 uses the field motion vectors of phase 1 358 todetermine whether a scene change exists. Depending on whether there is ascene change or not, the encoder may alter the encoding process, asexplained below. 3:2 pull-down detection component 362 uses the fieldmotion vectors outputted from phase 1 358 to determine whether arepeated field exists in an inputted frame. If a repeated field exists,the eliminate repeated field component 364 may eliminate the repeatedfield using any of several known methods, including removing the fieldand inserting a reference to a previous field, and averaging the twoidentical fields to improve the image quality.

[0060] The controller, including motion estimation phase 2 366,finalizes this encoding process. Phase 2 of the motion estimationprocess 366 determines the remaining motion vectors, using areconstructed frame 368 as a reference, which refers to a reconstructedframe occurring before the frame which is being encoded. In oneembodiment, the phase 1 uses frames from the original video sequence todetermine motion vectors. Using original frames to compute motionvectors can lead to more accurate motion vectors because the originalframes are not deteriorated. Phase 2 can use reconstructed framesbecause the decoder has no information about the original frames, so thephase 2 motion estimation would need to use reconstructed frames toavoid error drifting. The controller including motion estimation phase 2366 outputs motion vectors 370.

[0061] This approach not only has effects of mixing reference to theoriginal frames and the reconstructed frames, but greatly reduces theoverall computational load, especially when repeated fields aredetected. In addition, since phase-one motion estimation is separatedfrom the encoding process, it can be executed in the same order as inputframes. Thus, this two-phase motion estimation structure will notincrease the complexity of the encoder.

[0062]FIGS. 4, 5, 6, 7, 8, and 9 illustrate timing considerations whenusing a motion estimator according to some embodiments. The followingtiming diagrams explain the operation of a motion estimator only inspecific circumstances, and are meant only as examples of the operationof a motion estimator. The following diagrams may be used to explain thetiming considerations of the process of FIG. 1.

[0063] Motion estimation with the mode decision to determine the bestmotion vector for each macroblock is one of the most computationintensive processes that an encoder must complete. As a result, aprocessor must take timing into account when the motion estimationprocess is modified, for example, when a repeated field is detected andthe second phase of the motion estimation becomes unnecessary. Thefollowing timing diagrams illustrate such considerations.

[0064] In one embodiment, a video encoder normally operates thetop-field-first condition. That is, when using interlaced frames, eachframe having a top field and a bottom field, the top field is encodedbefore the bottom field. In the bottom-field-first condition, the bottomfield of the frame is encoded first. As explained below, it is possiblefor the encoder change from the top-field-first condition to thebottom-field-first condition.

[0065]FIG. 4 is a timing diagram for a video sequence without scenechange or repeated fields. Frames 401, 402, 403, 404, 405, 406, 407,408, and 409 are to be encoded. Frames 401 through 409 are to be shownsequentially. Here, frame 408 is an I-frame. At time 410, the motionestimator initiates with two phase one motion estimations in order toapply 3:2 pull-down detection. This initiation puts the first phase twoframes ahead of the second phase. For example, frame 404 is processed inthe first phase of the motion estimator during the same cycle that frame402 is processed in the second phase. Encoding of a frame is completedwhen the second phase and final encoding operations are completed, soafter second phase motion estimation for frame 401 is completed at time411, frame 401 has been fully encoded. Then, the motion estimatorregularly performs the following steps in a cycle: phase one motionestimation for frame 404 (at time 412), scene change detection (at time413), 3:2 pull-down detection (at time 414), and phase two motionestimation and final encoding for frame 402 (at time 415).

[0066] Two frames before the next I-frame 408 is fully encoded at time418, at time 416 phase one for frame 409 is completed. During thiscycle, phase two for frame 406 is completed, so it would be expectedthat phase one for frame 408 would be executed because of the two framedelay. However, frame 408 is an I-frame, and as such has no motionvectors, and therefore does not require phase one motion estimation. So,the frame after the I-frame, frame 409, is encoded at time 416.Additionally, since there are no motion vectors for frame 408, scenechange and 3:2 pull-down detections are not performed. At time 418, anextra frame is encoded, so at time 419, the two frame delay and thecycle return to normal.

[0067]FIG. 5 is a timing diagram for a video sequence with repeatedfields. Frames 501, 502, 503, 504, 505, 506, 507, and 508 are to beencoded in sequence. Here, repeated fields can be encoded into the sameframe as the field they are repeated from. So, frame 504 has threefields, field 510 is a repeat of field 509. Similarly, frame 506 hasthree fields, field 512 is a repeat of field 511. When encoding threefields into one frame, there will be a time gain. For example, at time513, when encoding frame 504, there is extra time because the field 510does not need to be encoded again. This extra time can be used toimprove picture quality by taking an average of the two essentiallyidentical fields 509 and 510 for noise reduction or by doing motionestimation refinement. There is also extra time at time 514, whenencoding frame 506. As shown here, the extra time gained because of therepeated field is less for frame 506 than for 504. It is understood thatthe amount of time gained is variable, and will differ.

[0068] In one embodiment, after two repeated fields are removed, at time515, the first phase of the motion estimator needs to be executed twiceto keep up with the advance set of motion vectors.

[0069]FIG. 6 is a timing diagram for a video sequence with an I-frame inbetween two repeated fields. Frames 601, 602, 603, 604, 605, 606, 607,and 608 are to be encoded in sequence. Here, frames 601 and 606 haverepeated fields, and frame 605 is an I-frame. Field 610 is a repeat offield 609, and is a top field, whereas field 612 is a repeat of field611, and is a bottom field. In one embodiment, the encoder would beoperating in a top-field-first condition. However, when there is arepeated field as with frame 601, the bottom field 613 of frame 602 willbe encoded first, and the top field 614 second, and the encoder will berunning in a bottom-field-first condition. Normally, another frame witha repeated field would intervene and return the process to encoding thetop field first. I-frame 605 occurs before top-field-first encoding canbe resumed. So, to remedy this problem, the encoder can simply encodethe bottom field 615 of I-frame 605 first.

[0070]FIG. 7 is a timing diagram for a video sequence with a scenechange during a top-field-first situation. Frames 701, 702, 703, 704,705, 706, 707, and 708 are to be encoded in sequence. FIG. 14 furtherillustrates the sequence running in a top-field-first situation. Thereis a scene change detected between frames 702 and 703. Ideally, then,frame 703 would be encoded as an I-frame. There are concerns about thespeed of variable length encoding (VLE), and because there is low visualsensitivity before and after the scene change, the I-frame can bepostponed until frame 705. Also, frames 702 and 703 can be encoded asP-frames without adverse effect.

[0071]FIG. 8 is a timing diagram for a video sequence with a scenechange during a bottom-field-first situation. Frames 801, 802, 803, 804,805, 806, 807, and 808 are to be encoded in sequence. Frame 803 has arepeated field, and this causes the sequence to begin encodingbottom-field-first starting with frame 804. Further, there is a scenechange detected between frames 804 and 805. This may cause an extrafield 809, the bottom field of frame 804, to occur which cannot beencoded with frame 804, since frame 804 has already been encoded, andwhich cannot be encoded with frame 805, since frame 805 is a differentscene. This can be remedied by replacing field 809 with the bottom field810 of frame 805, and encoding frame 805 as a frame with a repeatedfield. The remaining fields can then be encoded as top-field-first.

[0072]FIG. 9 illustrates another embodiment of an encoder. In thisembodiment, the encoder is capable of encoding all three types of MPEGframes—I-frames, P-frames, and B-frames. However, it is understood thatthe following flow diagram represents the operation of only one specificembodiment, and that other embodiments may exist. This embodiment of anencoder uses many of the same steps and processes as the encoderdescribed in FIG. 1. It uses a two-phase motion estimation, scene changedetection, and 3:2 pull-down detection. The process 900 encodes anentire GOP. The encoder operates in two stages—a start stage 901 and aprocess stage 902.

[0073] The start stage 901 includes initializing the encoder so that itcan begin normal operation on the GOP. The encoder receives an input ofvideo, and processes two frames in the first phase of motion estimationto provide the forward field motion vectors to complete the scene changedetection and 3:2 pull-down detection operations later in encoding.First, the first frame of video is prefiltered in block 903. Then, thefirst phase of motion estimation is completed for the first frame inblock 904. The first phase of motion estimation provides the fieldmotion vectors—the motion vectors that relate the two fields of the samepolarity, as explained above. In block 905, the second frame of video isprefiltered, and in block 906, first phase motion estimation isperformed on the second video frame.

[0074] Process stage 902 does the encoding for each GOP. Afterdetermining two sets of field motion vectors, the process stage beginsin block 907, where scene change detection is performed to determine ifthere is a scene change between the I-frame and the following frame. Inblock 908, 3:2 pull-down detection is to check whether there is a fieldrepetition after this I-frame. In blocks 909 and 910, scene changedetection and 3:2 pull-down detection is performed for the framefollowing the I-frame. In block 911, the I-frame is encoded. In block912, if the next to be coded frame is an I-frame, the process returns toblock 907. The procedures in blocks 907-911 are performed when a new GOPis going to be encoded.

[0075] In one embodiment, if a scene change is found either in the frameafter a predetermined I-frame, or two frames after this I-frame, thenthis I-frame may be encoded as a P-frame in order to save resources,since the full encoding of the I-frame will not be referenced if a newGOP will be started soon afterward. Further, if the frame immediatelyfollowing the I-frame has a repeated field, as would be detected by the3:2 pull-down detection, a repeated field flag can be set while encodingthe I-frame in block 911. According to one embodiment, a repeated fieldflag in an MPEG encoded video bitstream indicates to a decoder that arepeated field exists and the decoder needs to compensate for thatrepeated field.

[0076] In block 912, if the next frame is a B or P frame, encodingcontinues with block 913. In block 913, the next frame is prefiltered.To have two sets of motion vectors in advance for the detections, thefield motion vectors are determined in block 914 when the first phase ofmotion estimation is executed. In block 915, scene change detection isexecuted for the frame. If a scene change is detected, the encoder canstart a new GOP in the next frame to reflect the fact that the video hasa new scene. More detailed case studies of scene change detection can befound below. In block 916, 3:2 pull-down detection is performed todetect repeated fields for the next frame. If a repeated field isdetected, the repeated field can be encoded with the previous frame. Anencoder can eliminate a repeated field using a number of methods,including removing the field and inserting a reference to the field fromwhich it was repeated, or averaging the two repeated fields to obtainhigher quality video. In block 917, the P- or B-frame is encoded bycompleting the encoding of the motion vectors using the second phase ofmotion estimation. Once the encoding of frames is completed in block917, the process may begin again at block 912, until the GOP isfinished.

[0077] When a scene change is detected, the encoder must determine whatto do with the current frame and the following frames. The encoder couldencode the current frame as an I-frame beginning a new GOP, but if theencoder considers the human visual system, there may be a better way torespond to a scene change. Since the sensitivity of human visual systemdrops before and after a scene change happens, the pictures close to anew scene can be coded in lower quality to save processing resources.FIGS. 10 through 13 illustrate several examples of situations in which ascene change is detected. The specifics of scene change detection areexplained below.

[0078]FIG. 10 illustrates a video sequence having a scene change afteran I-frame and before a B-frame. In the video sequence 1000, the scenechange occurs after I-frame 1001 at frame 1002, a B-frame. The videosequence consists of top fields 1004, 1006, 1008, 1010, 1012, 1014, and1016, and bottom fields 1018, 1020, 1022, 1024, 1026, 1028, and 1030.Frame 1002 is comprised of fields 1010 and 1024. Further, sequence 1000has set of motion vectors 1032 relating fields 1004 and 1008, set ofmotion vectors 1034 relating fields 1004 and 1006, set of motion vectors1036 relating fields 1008 and 1012, set of motion vectors 1038 relatingfields 1008 and 1010, set of motion vectors 1040 relating fields 1018and 1022, set of motion vectors 1042 relating fields 1018 and 1020, setof motion vectors 1044 relating fields 1022 and 1026, and set of motionvectors 1046 relating fields 1022 and 1024.

[0079] When a scene change occurs immediately after an I-frame, it maybe advantageous to code the I-frame as a P-frame, because an I-frameoccupies considerably more space than a P-frame, and may not be veryuseful as a reference frame because of the scene change. The encoder maythem encode frame 1001 as a P-frame, and because frame 1001 is now aP-frame, frame 1001 needs motion vectors As a result, motion estimationmust be performed, resulting in sets of motion vectors 1032 and 1040.The I-frame can be delayed to a later P-frame, here frame 1050 wouldbecome an I-frame, since the motion vectors for frame 1050 have not yetbeen calculated, and it will save processing time to use the motionvectors which had already been calculated for P-frame 1048. In addition,encoder can allocate fewer resources for encoding frames 1002 and 1048due to the lack of sensitivity of the human visual system near a scenechange, and save the resources for the other more important frames.

[0080]FIG. 11 illustrates a video sequence having a scene changeoccurring two frames after an I-frame. A scene change is detected invideo sequence 1100 at frame 1102. An I-frame was originally scheduledto occur at frame 1104, but for the same reasons as above, the encodercan delay the I-frame. When frame 1104 is converted from an I-frame to aP-frame, sets of motion vectors 1106 and 1108 must be determined. TheI-frame may be delayed until P-frame 1110, and since no motion vectorshave been determined for frame 1110, no computation will be wasted.

[0081]FIG. 12 illustrates a video sequence having a scene changehappening at a B-frame. Here, unlike the situation in FIG. 10, the scenechange occurs at a B-frame, but not immediately after an I-frame. Invideo sequence 1200, the scene change occurs at frame 1202. I-frame maybe encoded at frame 1204, immediately after the scene change, sincethere is no recent I-frame. The next P-frame, frame 1204, can beconverted to an I-frame, and since no motion vectors had been determinedfor frame 1204, no computation is wasted.

[0082]FIG. 13 illustrates a video sequence having a scene changeoccurring at a P-frame. In video sequence 1300, the scene change occursat frame 1302. Since motion vectors have already been determined forframe 1302, the encoder may choose to delay the encoding of a new Iframe to frame 1308, the next P-frame.

[0083] When a repeated scene is detected using 3:2 pull-down detection,the encoder determines what to do with the current frame and thefollowing frames. FIGS. 14 through 16 illustrate several examples ofsituations in which a repeated field is detected.

[0084]FIG. 14 illustrates a video sequence having a repeated field in anI-frame. Video sequence 1400 comprises top fields 1402, 1404, 1406, and1408, and bottom fields 1410, 1412, 1414, and 1416. I-frame 1418comprises fields 1402, 1404, and 1410. Field 1404 is a repeated fieldfrom field 1402. Video sequence 1400 further has sets of motion vectors1420, 1422, 1424, and 1426.

[0085] A 3:2 pull-down inverse can be performed to remove the repeatedfield 1404. When the repeated field 1404 is removed, and replaced with areference to field 1402, the next frame 1428 will be encoded in abottom-frame-first condition. Further, frame 1430 will be referencing toframe 1418 for motion estimation. Because the set of motion vectors 1422will no longer be useful to the encoder, set of motion vectors 1432 mustbe established relating field 1408 and field 1402. The encoder can thencontinue to operate as normal, in a bottom-field-first situation untilnext repeat field is detected.

[0086]FIG. 15 illustrates a video sequence having a repeated field in aB-frame. Video sequence 1500 comprises top fields 1502, 1504, 1506, and1508, and bottom fields 1510, 1512, 1514, and 1516. B-frame 1518comprises fields 1504, 1506, and 1512. Field 1506 is a repeat of field1504. Video sequence 1500 also has sets of motion vectors 1520, 1522,1524, and 1526. The next frame in the sequence, frame 1528, will beencoded in the bottom-field-first situation.

[0087] Repeated field 1506 will be replaced with a reference to field1504. Because set of motion vectors 1520, which field 1506 was using forencoding, are no longer needed, they can be removed. Field 1506 wasreferring to field 1502 for motion estimation, but now that field 1506has been removed, field 1508 can refer to field 1502 using motion vector1530. Rearranging the motion vectors in this manner allows the encoderto keep the correct timing.

[0088]FIG. 16 illustrates a video sequence having a repeated field in aP-frame. Video sequence 1600 comprises top fields 1602, 1604, and 1606,and bottom fields 1608, 1610, and 1612. P-frame 1614 is comprised offields 1602, 1604, and 1608. Field 1604 is a repeated field of field1602. Because field 1604 will be removed and replaced with a referenceto field 1602, set of motion vectors 1616 relating field 1604 and 1602is no longer necessary. Instead, motion vector 1618 may be substituted,relating field 1606 to field 1602.

[0089] For any of the cases in FIGS. 14, 15, and 16, abottom-field-first situation would be processed in the same manner.

[0090] For this embodiment of an encoder, the first phase motionestimation can be executed a few frames in advance for future use inscene change detection and 3:2 pull-down detection. However, because ofvariable length encoding (VLE), a condition in which a certain length ofvideo does not necessarily occupy a constant amount of data storage,I-frame encoding requires extra time. To compensate for this, in oneembodiment, motion estimation for future frames is not determined duringI-frame encoding. Also, since, in one embodiment, the encoder needs toencode P-frames before B-frames, and as a result, there is a three framedelay for both I- and P-frame encoding.

[0091] For example, the encoder would begin by prefiltering a frame attime 0, and prefiltering another frame at time1. At time 2, the encoderprefilters a third frame, and does first phase motion estimation for thefirst frame that was prefiltered at time 0. At time 3, the encoderprefilters a fourth frame, and performs first phase motion estimationfor the second frame which was prefiltered at time 1. At time 4, theencoder prefilters a fifth frame, performs first phase motion estimationfor the third frame prefiltered at time 2, performs scene change and 3:2pull-down detection for the first frame, using motion vectors from thefirst phase motion estimation at time 3, and finished by encoding theframe, including the second phase motion estimation. At time 5, theentire cycle is repeated, and so on, until the next I-frame comes up inthe video sequence.

[0092]FIG. 17 is a block diagram of an encoder according to oneembodiment. Video frames enter the system at point 1702. For thefollowing description, it is assumed that the frame currently enteringthe encoder is frame k. At block 1704, the encoder checks whether theincoming frame k is scheduled to be encoded as an I-frame.

[0093] If frame k is an I-frame, processing moves on to block 1706,where the encoder sets variables is_new_scene and skip_detect to 0. Inone embodiment, variables is_new_scene and skip_detect are flags used byan encoding scheme to instruct a decoder to properly decode a videobitstream, is_new_scene tells the encoder whether the current framemarks the beginning of a new scene, if it is equal to 0, then thecurrent frame is not the beginning of a new scene, and if it is equal to1, then the current frame is the beginning of a new scene, skip_detectis a variable which tells the encoder whether or not to perform the 3:2pull-down and scene change detections, a value of 0 means that thedetections should be performed, and a value of 1 means that they shouldbe skipped. Further, in block 1706, the motion vector buffers for framesk+1 (the frame immediately following frame k) and k+2 are located. Theprocess moves to block 1708, where if the frame is in a top-field-firstsituation, the process will move on to block 1710, whereas if the frameis not in a top field first situation, the process will move to block1712, where the I-frame will be encoded, and the process will move backto point 1702.

[0094] At block 1710, scene change detection is performed for the framek+1. If there is a scene change, the process moves on to 1714, where thecurrent frame k, which is an 1-frame will be changed to a P-frame, andnew motion vectors will be determined for the frame, as was explained inFIG. 10. The process will then move on to block 1716, where the P-framewill be encoded, and the process will return to point 1702.

[0095] At block 1710, if there is no scene change, the process moves onto block 1718, where 3:2 pull-down detection is performed for theI-frame k. If a repeated field is detected, the process moves to block1720, where the pull-down handler is executed for the necessaryadjustment as was explained in FIG. 14, and moves to block 1712, wherethe I-frame is encoded, and finally returns to point 1702.

[0096] At block 1718, if there is no repeated field, the process movesto block 1722, where scene change is performed for the next frame. Ifthere is a scene change, the process continues to block 1724, where thescene change handler is called for the adjustment explained in FIG. 11,and the process moves on to block 1716 for encoding, and finally back topoint 1702.

[0097] At block 1722, if there is no scene change, the process continuesto block 1726, where 3:2 pull-down detection is performed on the nextframe. If there is a repeated field, the process continues to block1728, where the pull-down handler is executed for the necessaryadjustment explained in FIG. 15, and then proceeds to block 1712, wherethe I-frame is encoded, and finally returns to point 1702.

[0098] If, in block 1704, frame k is not an I-frame, the processcontinues to block 1730. In block 1730, the first phase of motionestimation is performed for future frames, and skip_detect is set to 0or 1. The process continues to block 1732, where if either is_new_sceneor skip_detect are equal to 1, the process continues to block 1716, andthe frame is encoded before returning to point 1702.

[0099] If, at block 1732, it is determined that both is_new_scene andskip_detect are equal to 0, then the process continues to block 1734. Atblock 1734, the encoder locates the first and second motion vectorbuffers for detection, and gets the picture type for the detectionframe. The process then continues to block 1736, where it is determinedwhether the variable is_frame_gain is equal to 1 or 0. If is_frame_gainis equal to 1, then there is a pair of repeated fields being detectedand the encoder gains one frame of time. The process continues to block1738, where a scene change detection is performed.

[0100] If a scene change is detected in block 1738, the process moves toblock 1740, where the scene change handler is run, the first phase ofmotion estimation for future frames is performed, and the skip_detectvariable is set. The process then continues to block 1742, whereis_frame_gain is set to 0 to reset the indicator of frame gain, andfinally the process returns to point 1702.

[0101] If at block 1738 it is determined that there is no scene changeat frame k, the process continues to block 1744, where the first phaseof motion estimation is performed for future frames, and the skip_detectvariable is set to 0 or 1 based on whether there are enough sets offield motion vectors for detections to be performed. The processcontinues to block 1746, where if skip-detect is equal to 1, the processcontinues to block 1742, and back to point 1702. If, at block 1746,skip_detect is equal to 0, the process continues to block 1748. At block1748, the next pair of sets of motion vectors and the picture codingtype for the next frame are retrieved, and the process continues toblock 1750.

[0102] At block 1750, a scene change detection is performed using themotion vector sets from block 1748. If a scene change is detected, theprocess continues to block 1752, where the scene change handler iscalled for the adjustment explained in FIGS. 12 and 13, and then theprocess continues to block 1742, and finally back to point 1702. Ifthere is no scene change at block 1750, the process continues to block1754, where 3:2 pull-down detection is executed. If there is a repeatedfield, the process continues to block 1756, where the pull-down handleris called for the necessary adjustment as explained in FIGS. 15 and 16,and then to block 1742, and finally back to point 1702. If there is norepeated field at block 1754, the process continues to block 1742, andfinally to point 1702.

[0103] At block 1736, if the is_frame_gain variable is equal to 0, theprocess continues to block 1758, where a scene change detection isperformed. If there is a scene change, the process continues to block1760 where the scene change handler is called for the adjustmentexplained in FIGS. 12 and 13, and to block 1716, where the frame isencoded, and finally returns to point 1702. If there is no scene changedetected in block 1758, the process continues to block 1762. At block1762, the 3:2 pull-down detection is executed to determine if there is arepeated field. If there is a repeated field, the process continues toblock 1764 where the pull-down handler is executed for the necessaryadjustment explained in FIGS. 15 and 16, and on to block 1716 for frameencoding, and finally back to point 1702. If there is no repeated fieldat block 1762, the process continues to block 1716 for frame encoding,and back to point 1702.

[0104] As discussed above the encoders described selectively detectrepeated fields using 3:2 pull-down detection. FIGS. 18a through 21explain methods for detecting repeated fields using 3:2 pull-downdetection according to one embodiment.

[0105]FIG. 18a illustrates two video frames and sets of motion vectorsrelating the two video frames. Video frame 1805 comprises two fields,top field 1810 and bottom field 1815. Similarly, video frame 1820similarly comprises two fields, top field 1825 and bottom field and1830. In this embodiment, video frames 1805 and 1820 are interlacedvideo frames, meaning that two fields make up each frame. However, it isunderstood that any configuration of video frames may be used.

[0106] Set of motion vectors 1835, 1840, 1845, 1850 and 1855 relatevideo frames 1805 and 1820. As explained above, a motion vector relatesthe motion of one block or region of a field to another field.Therefore, there are typically several motion vectors that relate twofields. Here, the arrows representing sets of motion vectors 1835, 1840,1845, 1850, and 1855 may actually represent several motion vectors.

[0107] Set of motion vectors 1835 describes the relationship between topfield 1810 and top field 1 825. Set of motion vectors 18 40 describesthe relationship between top field 1810 and bottom field 1830. Set ofmotion vectors 1845 describes the relationship between first video frame1805, and second video frame 1820. Set of motion vectors 1850 describesthe relationship between bottom field 1815 and top field 1825. Set ofmotion vectors 1855 describes the relationship between bottom field 1815and bottom field 1830.

[0108] Since top field 1810 and top field 1825 are both the top fieldsin a frame, they are said to be of the same polarity. Likewise, bottomfield 1815 and bottom field 1830 are of the same polarity, since theyare both bottom fields. Further, the sets of motion vectors 1835 and1855 are known as field motion vectors, since they relate two fields ofthe same polarity.

[0109] When encoding, it is possible to use motion vectors to determinewhether a repeated field exists. For example, if field 1825 were arepeat of field 1810, then theoretically, all members of the set ofmotion vectors 1835 would have a magnitude of zero, since there would beno changes to track between the two fields. However, there is alwayssome noise in any video system and some or all of the members of setmotion vector 1835 may have some non-zero magnitude. However, if field1825 is a repeat of field 1810, the sum of the magnitudes of the membersof the set of motion vectors 1835 could be much less than will be thesum of the magnitudes of the members of the set of motion vectors 1855relating fields 1830 and 1815, which are not repeated.

[0110] The following describes a process to detect repeated fields in ageneral manner, with more specific examples following. To find repeatedfields, set of motion vectors 1855 and 1835 can be compared. A ratio ofthe sum of the magnitudes the members of set of motion vectors 1835 andthe sum of the magnitudes the members of set of motion vectors 1855 canbe compared to a threshold value. The threshold value accounts fornoise, and can be a heuristically determined value. The detail will beexplained later. If field 1825 is found to be a repeated field of field1810, then it is only necessary to encode field 1825 by referring to theearlier field 1810. Additionally, one frame could be encoded with threefields, including the repeated field, and could further includereferences to account for the proper timing. Thusly, the encoder maysave the bits that would be used to describe the motion vectors and theresidual errors and the processing that would be necessary toreconstruct field 1825 and instead include only a reference to theearlier field 1810. Further, little additional processing has to be doneusing this method, since these motion vectors have to be calculated aspart of the encoding process anyway.

[0111] To determine a repeated field, an encoder using motion vectors toexecute 3:2 pull-down detection does not need to know the noise level inthe system. Current methods of detecting repeated fields need todetermine a noise level to detect whether there is a repeated field.Because motion vectors alone are enough to determine whether a repeatedfield exists, the additional computations of determining noise levelscan be avoided.

[0112] As a result, if the field is repeated, it can be removed from theencoding process and the sets of motion vectors 1840, 1845 and 1850 neednot be calculated. Consequently, a large amount of processing time andstorage space may be saved.

[0113]FIG. 18b illustrates a sequence of frames operating in atop-field-first condition. In a top-field-first condition, the top fieldof a frame is display before the bottom field. Sequence of frames 1860has frame 1861 having a top field 1862 and a bottom field 1863. Sincethe encoder is operating in a top-field-first condition, the top field1862 will come before the bottom field 1863. Top fields 1864 and 1865follow frame 1861. Set of motion vectors 1866 is a set of motion vectorsbetween fields 1862 and 1864, which may also be referred to as V_(0,1)Similarly, set of motion vectors 1867, which is between fields 1864 and1865, may be referred to as V_(1,2).

[0114]FIG. 18c illustrates a sequence of frames with a repeated field ina top-field-first condition. Sequence of frames 1870 has frame 1871having a top field 1872, bottom field 1873, and repeated top field 1874.Following frame 1871 are bottom field 1875 and top field 1876. Set ofmotion vectors 1877 relates top field 1872 and repeated field 1874, andmay also be referred to as V_(0,1). Similarly, set of motion vectors1878 relates bottom field 1873 and bottom field 1875, and may bereferred to as Λ_(0,1), the Λ indicating that it is a set of motionvectors relating a pair of fields of the opposite polarity as the set ofmotion vectors represented by V Also, set of motion vectors 1879 relatestop field 1874 and top field 1876, and may also be referred to asV_(1,2).

[0115]FIG. 18d illustrates a sequence of frames operating in abottom-field-first condition. In a bottom-field-first condition, thebottom field of a frame is display before the top field. Sequence offrames 1880 has frame 1881 having a top field 1882 and a bottom field1883. Since the encoder is operating in a bottom-field-first condition,the bottom field 1883 will come before the top field 1882. Bottom fields1884 and 1885 follow frame 1881. Set of motion vectors 1886 is a set ofmotion vectors between fields 1882 and 1884, which may also be referredto as V_(0,1). Similarly, set of motion vectors 1887, which is betweenfields 1884 and 1885, may be referred to as V_(1,2).

[0116]FIG. 18e illustrates a sequence of frames with a repeated field ina bottom-field-first condition. Sequence of frames 1890 has frame 1891having a top field 1892, bottom field 1893, and repeated bottom field1894. Following frame 1891 are top field 1895 and bottom field 1896. Setof motion vectors 1897 relates bottom field 1893 and repeated field1894, and may also be referred to as V_(0,1). Similarly, set of motionvectors 1898 relates top field 1892 and top field 1895, and may bereferred to as Λ_(0,1), the Λ indicating that it is a set of motionvectors relating a pair of fields of the opposite polarity as the set ofmotion vectors represented by V. Also, set of motion vectors 1899relates bottom field 1894 and bottom field 1896, and may also bereferred to as V_(1,2).

[0117] The detection of repeated fields can be represented by thefollowing equations. In this first equation, repeated fields can bedetected in a video sequence containing I and P frames:${{{If}\quad \frac{{\sum{V_{0,1}}_{c}} + ɛ}{{\sum{V_{1,2}}_{c}} + ɛ}} < {\tau \quad {and}\quad \frac{{\sum{_{0,1}}_{c}} + ɛ}{{\sum{V_{0,1}}_{c}} + ɛ}} > \frac{1}{\tau}},$

[0118] Then field t+1 is a repeated field from field t for top-fieldfirst cases, or field b+1 is a repeated field from field b for bottomfield cases. Here, t and b can be the top and bottom fields of a frameat time 0. | |_(c) means that the absolute values for all components ofthe vectors are used. ε is a small positive number to avoid falsedetection and division by zero. v_(i,j) represents the set of motionvectors between the fields t+i and t+j or between the fields b+i andb+j. Λ_(i,j) represents the motion vectors between the fields having theopposite polarity from the fields represented by V_(i,j) or in otherwords, if the fields represented by V_(i,j) are the top fields, then thefields of opposite polarity are the bottom fields and vice versa . . . τis the predetermined threshold.

[0119]FIGS. 19a, 19 b, 19 c, 19 d, 19 e, and 19 f illustrate a sequenceof frames having repeated fields according to one embodiment. Thefollowing equations may be used to detect repeated fields where thereare B frames in a video stream. In the following equations, thevariables are the same as above, but also include two thresholds τ1 andτ2, where τ1 should be smaller than 1, and τ2 should be larger than 1,or about 2, and k, where k is the frame distance between the referencefield and the target field, in the following illustrations, k=2.Further, when referring to a set of vectors between two fields, forexample, the notation V_(t0→t1) represents the set of motion vectorsbetween a first top field (field t0) and a second top field (field t1),the field t1 in the frame immediately proceeding the frame having thefield t0. Likewise, the notation V_(t0→t2) refers to a set of motionvectors between a field t0, and a field t2, the field t2 coming twoframes after the field t0. A field b0 would refer to the bottom field ofthe frame of field t0.

[0120] The following equation can be used to determine if there is arepeated field in a situation in which a video sequence is operatingtop-field first, and the current frame is an I frame:${{If}\quad \frac{\sum{V_{{t0}\rightarrow{t1}}}_{c}}{\sum{V_{{t1}\rightarrow{t2}}}_{c}}} < {\tau_{1}\quad {and}\quad \frac{\sum{V_{{b0}\rightarrow{b1}}}_{c}}{\sum{V_{{t0}\rightarrow{t1}}}_{c}}} < \tau_{2}$

[0121] then field t1 is repented

[0122] Such a situation is illustrated in FIG. 19a Video sequence 1900has a current frame with a repeated field 1901, top fields 1902, 1903,1904, and 1905, and bottom fields 1906, 1907, 1908, and 1909. Fields1902 and 1903 are repeated, and frame 1901 is made up of fields 1902,1903 and 1906. In the equations, fields 1902, 1903, 1904, and 1905correspond to fields t0, t1, t2, and t3, respectively. Similarly, fields1906, 1907, 1908, and 1909 correspond to fields b0, b1, b2, and b3,respectively. Since fields 1903 and 1907 are originally scheduled as aB-frame, which can not be used as a reference frame, top field 1904 hasmotion vectors referring to field 1902 rather than field 1903.

[0123] If the above equation is true, then field 1903 is a repeat offield 1902, and it can be encoded as such. Further, the encoder shouldbegin encoding bottom-field-first, starting with field 1907. Thus, theframe following frame 1901 will be comprised of fields 1904 and 1907.

[0124] The following equation can be used to determine if there is arepeated field in a situation in which a video sequence is operatingtop-field first, and the current frame is a B frame:${{{if}\quad \frac{{{\sum{V_{{t0}\rightarrow{t1}}}_{c}} - {\sum{V_{{t0}\rightarrow{t2}}}_{c}}}}{\sum{V_{{t0}\rightarrow{t1}}}_{c}}} < \tau_{1}},\quad {\frac{{{\sum{V_{{b0}\rightarrow{b1}}}_{c}} - {\sum{V_{{b0}\rightarrow{b2}}}_{c}}}}{{{\sum{V_{{t0}\rightarrow{t1}}}_{c}} - {\sum{V_{{t0}\rightarrow{t2}}}_{c}}}} > \tau_{2}},{{{and}\quad \frac{\sum{V_{{t2}\rightarrow{t3}}}_{c}}{\frac{1}{k}{\sum{V_{{t0}\rightarrow{t2}}}_{c}}}} < \tau},$

[0125] then field t2 is repeated

[0126] Such a situation is illustrated in FIG. 19b. Video sequence 1910has a current frame 1911 with a repeated field, top fields 1912, 1913,1914, and 1915, and bottom fields 1916, 1917, 1918, and 1919. Fields1912, 1913, 1914, and 1915 correspond to fields t0, t1, t2, and t3,respectively. Similarly, fields 1916, 1917, 1918, and 1919 correspond tofields b0, b1, b2, and b3. Fields 1914 and 1913 are repeated, and frame1911 is made up of fields 1913, 1914 and 1917. Since fields 1913 and1917 are originally scheduled as a B-frame, top field 1914 has motionvectors referring to field 1912 rather than field 1913.

[0127] If the above equation is true, then field 1914 is a repeat offield 1913, and it can be encoded as such. Further, the encoder shouldbegin encoding bottom-field-first, starting with field 1918.

[0128] The following equation can be used to determine if there is arepeated field in a situation in which a video sequence is operatingtop-field first, and the current frame is a P frame:${{if}\quad \frac{\sum{V_{{t2}\rightarrow{t3}}}_{c}}{\frac{1}{k}{\sum{V_{{t0}\rightarrow{t2}}}_{c}}}} < {\tau_{1}\quad {and}\quad \frac{{\sum{V_{{f0}\rightarrow{f1}}}_{c}} + ɛ}{{\sum{V_{{f1}\rightarrow{f2}}}_{c}} + ɛ}} > \tau_{1}$

[0129] Such a situation is illustrated in FIG. 19c. Video sequence 1920has a current frame 1921 with a repeated field, top fields 1922, 1923,1924, and 1925, and bottom fields 1926, 1927, 1928, and 1929. Fields1922, 1923, 1924, and 1925 correspond to fields t0, t1, t2, and t3respectively. Similarly, fields 1926, 1927, 1928, and 1929 correspond tofields b0, b1, b2, and b3 respectively. Fields 1924 and 1925 arerepeated, and frame 1921 is made up of fields 1924, 1925 and 1928.Because field 1925 is a repeat of field 1924, there is no need to createa set of motion vectors relating fields 1925 and 1924 other than thefield motion vectors, and the encoder need only make a note that fields1925 and 1924 are the same.

[0130] If the above equation is true, then field 1925 is a repeat offield 1924, and it can be encoded as such. Further, the encoder shouldbegin encoding bottom-field-first, starting with field 1929.

[0131] The following equation can be used to determine if there is arepeated field in a situation in which a video sequence is operatingbottom-field first, and the current frame is an I frame:${{if}\quad \frac{\sum{V_{{b0}\rightarrow{b1}}}_{c}}{\sum{V_{{b0}\rightarrow{b2}}}_{c}}} < {\tau_{1}\quad {and}\quad \frac{\sum{V_{{t0}\rightarrow{t1}}}_{c}}{\sum{V_{{b0}\rightarrow{b1}}}_{c}}} > \tau_{2}$

[0132] then field b1 is repented

[0133] Such a situation is illustrated in FIG. 19d. Video sequence 1930has a current frame 1931 with a repeated field, top fields 1932, 1933,1934, and 1935, and bottom fields 1936, 1937, 1938, and 1939. Fields1932, 1933, 1934, and 1935 correspond to fields t0, t1, t2, and t3,respectively. Similarly, fields 1935, 1936, 1937, and 1938 correspond tofields b0, b1, b2, and b3, respectively. Fields 1936 and 1937 arerepeated, and frame 1931 is made up of fields 1932, 1936 and 1937.Because field 1937 is a repeat of field 1936, there is no need to createa set of motion vectors relating fields 1937 and 1936 other than thefield motion vectors, but the encoder need only make a note that fields1937 and 1936 are the same.

[0134] If the above equation is true, then field 1937 is a repeat offield 1936, and it can be encoded as such. Further, the encoder shouldbegin encoding top-field-first, starting with field 1933.

[0135] The following equation can be used to determine if there is arepeated field in a situation in which a video sequence is operatingbottom-field first, and the current frame is a B frame:${{{{if}\quad \frac{{{\sum{V_{{b0}\rightarrow{b1}}}_{c}} - {\sum{V_{{b0}\rightarrow{b2}}}_{c}}}}{\sum{V_{{b0}\rightarrow{b1}}}_{c}}} < \tau_{1}},{\frac{{{\sum{V_{{t0}\rightarrow{t1}}}_{c}} - {\sum{V_{{t0}\rightarrow{t2}}}_{c}}}}{{{\sum{V_{{b0}\rightarrow{b1}}}_{c}} - {\sum{V_{{b0}\rightarrow{b2}}}_{c}}}} > {\tau_{2}\quad {and}}}}\quad$

[0136] Σ|V_(b0→b2)|_(c)<Σ|V_(t0→t2)|_(c) then field b2 is repeated

[0137] Such a situation is illustrated in FIG. 19e. Video sequence 1940has a current frame 1941 with a repeated field, top fields 1942, 1943,1944, and 1945, and bottom fields 1946, 1947, 1948, and 1949. Fields1942, 1943, 1944, and 1945 correspond to fields t0, t1, t2, and t3respectively. Fields 1946, 1947, 1948, and 1949 correspond to fields b0,b1, b2, and b3 respectively. Fields 1948 and 1947 are repeated, andframe 1941 is made up of fields 1943, 1947 and 1948. Because field 1948is a repeat of field 1947, the encoder need only make a note that fields1948 and 1947 are the same.

[0138] If the above equation is true, then field 1948 is a repeat offield 1947, and it can be encoded as such. Further, the encoder shouldbegin encoding top-field-first, starting with field 1944.

[0139] The following equation can be used to determine if there is arepeated field in a situation in which a video sequence is operatingbottom-field first, and the current frame is a P frame:${{if}\quad \frac{\sum{V_{{b2}\rightarrow{b3}}}_{c}}{\frac{1}{k}{\sum{V_{{b0}\rightarrow{b2}}}_{c}}}} < {\tau_{1}\quad {and}\quad \frac{\sum{V_{{t2}\rightarrow{t3}}}_{c}}{\sum{V_{{b2}\rightarrow{b3}}}_{c}}} > \tau_{2}$

[0140] then field b3 is repeated

[0141] Such a situation is illustrated in FIG. 19f. Video sequence 1950has a current frame 1951 with a repeated field, top fields 1952, 1953,1954, and 1955, and bottom fields 1956, 1957, 1958, and 1959. Fields1952, 1953, 1954, and 1955 correspond to fields t0, t1, t2, and t3.Similarly, fields 1956, 1957, 1958, and 1959 correspond to fields b0,b1, b2, and b3. Fields 1958 and 1959 are repeated, and frame 1951 ismade up of fields 1954, 1958 and 1959. Because field 1959 is a repeat offield 1958, there is no need to create a set of motion vectors relatingfields 1959 and 1958 other than the field motion vectors, but theencoder need only make a note that fields 1959 and 1958 are the same.

[0142] If the above equation is true, then field 1959 is a repeat offield 1958, and it can be encoded as such. Further, the encoder shouldbegin encoding top-field-first, starting with field 1955.

[0143]FIG. 20 illustrates two frames and their associated motion vectorsaccording to one embodiment. Video frame 2005 comprises top field 2010and bottom field 2015. Video frame 2020 comprises top field 2025 andbottom field 2030. Field 2010 has associated motion vectors 2035relating field 2010 to a previous reference field with the samepolarity. Similarly, field 2015 has motion vectors 2040 also relatingthe field to a previous reference field with the same polarity. Field2025 has motion vectors 2045 relating field 2025 to field 2010. Finally,field 2030 has motion vectors 2050 relating field 2030 to field 2015.

[0144] Motion vectors 2035, 2040, 2045 and 2050 represent the movementof blocks of fields 2010, 2015, 2025 and 2030. Each of the fields isdivided into blocks, each block comprising a certain number of pixels.The movement of a block from field to field is tracked by the motionvectors.

[0145] Field 2025 is a repeated field of field 2010. This can bedetermined because of the relatively small magnitude and number ofmotion vectors 2045 as compared to the number of and magnitude of motionvectors 2050. Because there are relatively few motion vectors 2045, thisindicates that the blocks of field 2010 have not moved much relative tothe blocks of field 2025 any more than can be accountable to noise.Thus, motion vectors 2045 may be used to determine whether or not field2025 is a repeat of field 2010. Further, as above, in comparing motionvectors 2050 to motion vectors 2045, it can be determined whether field2025 is either repeated or whether the whole frame 2020 is the same asthe previous frame 2005) because there is a still image.

[0146]FIG. 21 is a flow diagram illustrating the process of detectingrepeated fields according to one embodiment. This process is also knownas 3:2 pull-down detection. At block 2105, a video device receives afirst frame and a second frame. In one embodiment, each frame isinterlaced, that is each frame is made up of two or more separate imagesor fields. A typical interlacing scheme has two fields for each frame, afirst field and a second field, typically the first field being a topfield and the second field being a bottom field, each field havingalternating horizontal lines. At block 2110, a motion estimatordetermines a first set of motion vectors. Here, the first set of motionvectors is between the first field of the first frame and the firstfield of the second frame. At block 2115, a second set of motion vectorsis determined between the second field of the first frame and the secondfield of the second frame. At block 2120, these field motion vectors areused to determine whether one of the fields in the second frame issubstantially similar to the corresponding field in the first frame. Thespecific manner for determining the repeated fields is detailed in FIGS.19a, 19 b, 19 c, 19 d, 19 e, and 19 f. Generally, the field motionvectors can be used to determine this because if a motion vector isdetermined between two identical fields, then the magnitude of theresulting motion vector will theoretically be zero. However, inreal-world applications there is always some noise and some differencebetween the fields. If one of the two sets of motion vectors has a muchsmaller magnitude than the other, it can be said that there is arepeated field. Since motion vectors have to be determined as part ofthe encoding process anyway, efficiency can be increased by using thosemotion vectors to determine where repeated fields are, and then thoserepeated fields need not be physically encoded, but rather can use areference to the earlier field from which they were repeated.

[0147]FIGS. 22a through 24 explain a method for detecting scene changesin a video sequence according to one embodiment.

[0148]FIGS. 22a, 22 b, and 22 c illustrate a sequence of framescontaining a scene change according to one embodiment. A scene changeoccurs in a video sequence where the image in the video transitions fromone scene to another distinguishably distinct scene. If an encoder knowswhere a scene change is, the encoder can begin a new Group of Pictures(GOP) with the new scene. Current methods for detecting a scene changerequire burdensome amounts of computation, as well as long delays, andtherefore cannot be encoded real time using many current encoders.

[0149] According to one embodiment of the present invention, a videoencoder can detect scene changes using sets of motion vectors which mustbe estimated as part of the normal encoding process. The encoder may usetwo sets of motion vectors, one set relating the top field of a firstframe and the top field of a second frame, and another set relating thebottom field of a first frame and the bottom field of a second frame.Because the encoder can detect a scene change using these two sets ofmotion vectors, if a scene change is found, the estimation of remainingthree sets of motion vectors (as explained with respect to FIG. 18) andthe mode decision for determining the final motion vectors, need nolonger be processed, as the encoder may encode the current frame as an Iframe beginning a new GOP. Thus, the encoder uses less resources thanprevious approaches to detect the scene change, and once the scenechange is detected, more computational load can be avoided such as themode decision for determining the best motion vector from fivecandidates for each corresponding data block.

[0150] In one embodiment, the following three equations determinewhether a scene change has happened following I-, B-, and P-frames. Thecalculations for top-field-first and for bottom-field-first are thesame, so the top-field-first situation is shown below. If the encoder isrunning in bottom-field-first, the scene change calculation can beperformed by simply replacing the references in the equations to topframes with references to bottom frames, and vice versa. The variablesare the same as those explained above in FIGS. 19a through 19 f, withthe addition of num_mblock, which refers to the number of macroblocks ina frame. The threshold is used to avoid false detection due to smallmotion, field/frame repetition, or a main object having a largemovement.

[0151] These equations determine whether a frame following an I-frame ina top-field-first situation is the site of a scene change:${{{if}\quad \frac{{\sum{V_{{t0}\rightarrow{t1}}}_{c}} + ɛ}{{{{\sum{V_{{t0}\rightarrow{t1}}}_{c}} - {\sum{V_{{t0}\rightarrow{t2}}}_{c}}}} + ɛ}} > \tau_{1}},\quad {\frac{{\sum{V_{{b0}\rightarrow{b1}}}_{c}} + ɛ}{{{{\sum{V_{{b0}\rightarrow{b1}}}_{c}} - {\sum{V_{{b0}\rightarrow{b2}}}_{c}}}} + ɛ} > \tau_{1}},{and}$

[0152] Σ|V_(t0→t1)|_(c)>τ2*num_mblock then scene change at frame t1/b1

[0153] This situation is illustrated in FIG. 22a. Video sequence 2200has top fields 2201, 2202, and 2203, and bottom fields 2204, 2205, and2206. In the equations, fields 2201, 2202, and 2203 are referred to asfields t0, t1, and t2, respectively. Likewise, fields 2204, 2205, and2206 are referred to as fields b0, b1, and b2 respectively. Set ofmotion vectors 2207 relates field 2203 to field 2201, set of motionvectors 2208 relates field 2202 to field 2201, set of motion vectors2209 relates field 2206 to field 2204, and set of motion vectors 2210relates field 2205 to field 2204. Here, if all of the above equationsare satisfied, then a scene change happens at the frame comprisingfields 2202 and 2205.

[0154] These equations determine whether a frame following an B-frame ina top-field-first situation is the site of a scene change:${{{if}\quad \frac{{\sum{V_{{t0}\rightarrow{t2}}}_{c}} + ɛ}{{\sum{V_{{t0}\rightarrow{t1}}}_{c}} + ɛ}} > \tau_{1}},{\frac{{\sum{V_{{b0}\rightarrow{b2}}}_{c}} + ɛ}{{\sum{V_{{b0}\rightarrow{b1}}}_{c}} + ɛ} > \tau_{1}}$

[0155] and Σ|V_(t0→t2)|_(c)>τ2*num_mblock then scene change at t2/b2This situation is illustrated in FIG. 22b. Video sequence 2220 has topfields 2221, 2222, and 2223, and bottom fields 2224, 2225, and 2226. Inthe equations, fields 2221, 2222, and 2223 are referred to as fields t0,t1, and t2, respectively. Likewise, fields 2224, 2225, and 2226 arereferred to as fields b0, b1, and b2, respectively. Set of motionvectors 2227 relates field 2223 to field 2221, set of motion vectors2228 relates field 2222 to field 2221, set of motion vectors 2229relates field 2226 to field 2224, and set of motion vectors 2230 relatesfield 2225 to field 2234. Here, if all of the above equations aresatisfied, then a scene change happens at the frame comprising fields2223 and 2226.

[0156] These equations determine whether a frame following an P-frame ina top-field-first situation is the site of a scene change:${{{if}\quad \frac{{\sum{V_{{t2}\rightarrow{t3}}}_{c}} + ɛ}{{\sum{V_{{t0}\rightarrow{t2}}}_{c}} + ɛ}} > \tau_{1}},{\frac{{\sum{V_{{b2}\rightarrow{b3}}}_{c}} + ɛ}{{\sum{V_{{b0}\rightarrow{b2}}}_{c}} + ɛ} > \tau_{1}}$

[0157] and Σ|V_(t2→t3)|_(c)>τ2*num_mblock then scene change at t3/b3

[0158] This situation is illustrated in FIG. 22c. Video sequence 2240has top fields 2241, 2242, 2243, and 2244, and bottom fields 2245, 2246,2247, and 2248. In the equations, fields 2241, 2242, 2243, and 2244 arereferred to as fields to, t1, t2, and t3, respectively. Likewise, fields2245, 2246, 2247, and 2248 are referred to as fields b0, b1, b2, and b3,respectively. Set of motion vectors 2249 relates field 2243 to field2241, set of motion vectors 2250 relates field 2244 to field 2243, setof motion vectors 2251 relates field 2247 to field 2245, and set ofmotion vectors 2252 relates field 2248 to field 2247. Here, if all ofthe above equations are satisfied, then a scene change happens at theframe comprising fields 2244 and 2248.

[0159]FIG. 22d illustrates an interlaced video sequence having a scenechange. Video sequence 2260 has top fields 2261, 2262, and 2263, andbottom fields 2264, 2265, and 2266. Set of motion vectors 2267 relatesfields 2261 and 2262, set of motion vectors 2268 relates fields 2262 and2263, set of motion vectors 2269 relates fields 2264 and 2265, and setof motion vectors 2270 relates fields 2265 and 2266. According to oneembodiment, a scene change can be found in this sequence using thefollowing equations:${{If}\quad \frac{\sum{V_{{t0}\rightarrow{t1}}}_{c}}{\sum{V_{{t1}\rightarrow{t2}}}_{c}}} > {\tau_{1}\quad {and}\quad \frac{\sum{V_{{b0}\rightarrow{b1}}}_{c}}{\sum{V_{{b1}\rightarrow{b2}}}_{c}}} > \tau_{1}$

[0160] and Σ|V_(t0→t1)|_(c)>(τ2*num_mblock) then scene change at framet1/b1.

[0161] The above equations use the same variables as the equations inFIGS. 22a, 22 b, and 22 c. Here, fields 2261, 2262, and 2263 correspondto fields t0, t1, and t2 respectively. Likewise, fields 2264, 2265, and2266 correspond to fields b0, b1, and b2, respectively. If all of theabove equations are true, then there is a scene change at the framecontaining fields 2262 and 2265.

[0162]FIG. 22e illustrates a progressive video sequence having a scenechange. In a progressive video sequence, there are no fields, only videoframes. Video sequence 2280 has frames 2281, 2282, and 2283. Set ofmotion vectors 2284 relates fields 2281 and 2282, and set of motionvectors 2285 relates fields 2282 and 2283. According to one embodiment,a scene change can be found using the following equations:${{If}\quad \frac{{\sum{V_{{f0}\rightarrow{f1}}}_{c}} + ɛ}{{\sum{V_{{f1}\rightarrow{f2}}}_{c}} + ɛ}} > \tau_{1}$

[0163] and Σ|V_(f0→f1)|_(c)>(τ2*num_mblock) then scene change at framef1.

[0164] The above equations use the same variables as the equations inFIGS. 22a, 22 b, and 22 c, with the addition of the variables f0, f1,and f2. Frame f1 corresponds to frame 2281, frame f2 corresponds toframe 2282, and frame f3 corresponds to frame 2283. If all of the aboveequations are true, then there is a scene change at frame 2282.

[0165]FIG. 23 illustrates sets of motion vectors in two frames in avideo sequence. Frame 2302 is immediately proceeded by frame 2304. Frame2302 is comprised of top field 2306 and bottom field 2308, and frame2304 is comprised of top field 2310 and bottom field 2312. Motionvectors 2314 illustrate the magnitude and direction of blocks of field2306 in relation to a reference frame immediately preceding frame 2302.Likewise, motion vectors 2316 illustrate the magnitude and direction ofblocks of field 2308 in relation to a reference frame immediatelypreceding frame 2302. Motion vectors 2314 and 2316 are relatively smallin magnitude. Thus, frame 2302 belongs to the same scene as the frameimmediately preceding it.

[0166] However, examining the motion vectors 2318 and 2320 of fields2310 and 2312, respectively, reveals that these motion vectors havemagnitudes that are much larger and directions that are much more randomthan the motion vectors 2314 and 2316 of fields 2306 and 2308. Becausemotion vectors indicate the motion of blocks of one frame related toanother, when there are very large and very random motion vectors forone frame, it can be concluded that that frame is not very similar tothe frame from which it depends. Therefore, it can be said that there isa scene change in a frame which has motion vectors having largemagnitude compared to the frame from which the current frame refers.

[0167]FIG. 24 is a flow diagram generally illustrating one embodiment ofa process described above for determining whether there is a scenechange. At block 2402, the motion vectors are determined for the fieldsof the same polarity between the current frame and the frame to whichthe current frame refers. At block 2404, the motion vectors for thecurrent frame are compared to the motion vectors of the previous frame.At block 2406, if it is determined that the ratio of the magnitudes forthe total sets of motion vectors of the current frame and the previousframe are greater than a threshold and the magnitudes of the motionvectors for the current frame are relatively large, then a scene changeis said to occur at the current frame.

[0168] The invention has been described in conjunction with the severalembodiments. It is evident that numerous alternatives, modifications,variations, and uses will be apparent to one skilled in the art in lightof the forgoing description.

What is claimed is:
 1. A method for identifying repeated fields in a video sequence comprising: determining a first set and second set of motion vectors from input video frames of a video sequence; identifying a repeated field by comparing the first set and second set of motion vectors to a first threshold.
 2. The method of claim 1, wherein the first set of motion vectors is between a first field of a first frame and a first field of a second frame, and the second set of motion vectors is between a second field of a first frame and a second field of a second frame.
 3. The method of claim 1, wherein input video frames further comprise a first frame having a first and second field, a second frame having a first and second field, and a third frame having a first and second field.
 4. The method of claim 3, wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, and the second set of motion vectors is between the second field of the first frame and the second field of the second frame, the method further comprising: determining a third set of motion vectors between the first field of the second frame and the first field of the third frame; and wherein identifying further comprises comparing a first ratio of the first set of motion vectors and the second set of motion vectors to the first threshold, comparing a second ratio of the first set of motion vectors and the third set of motion vectors to the first threshold, and identifying a repeated field if the first ratio and the second ratio are less than the first threshold.
 5. The method of claim 3, wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, the second set of motion vectors is between the second field of the first frame and the second field of the second frame; said method further comprising determining a third set of motion vectors between the first field of the first frame and the first field of the third frame; and wherein identifying further comprises comparing a first ratio of the first set of motion vectors and the third set of motion vectors to the first threshold, comparing a second ratio of the second set of motion vectors and the first set of motion vectors to a second threshold, and identifying a repeated field if the first ratio is less than the first threshold and the second ratio is greater than the second threshold.
 6. The method of claim 3, wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, the second set of motion vectors is between the second field of the first frame and the second field of the second frame; said method further comprising determining a third set of motion vectors between the first field of the first frame and the first field of the third frame and a fourth set of motion vectors between the second field of the first frame and the second field of the third frame; and wherein identifying further comprises comparing a first ratio of the sum of the first set of motion vectors minus the third set of motion vectors and the first set of motion vectors to the first threshold, comparing a second ratio of the sum of the second set of motion vectors minus the fourth set of motion vectors and the sum of the first set of motion vectors minus the third set of motion vectors to a second threshold, comparing the third set of motion vectors to the fourth set of motion vectors, and identifying a repeated field if the first ratio is less than the first threshold, the second ratio is greater than the second threshold, and the magnitude of the third set of motion vectors is less than the magnitude of the fourth set of motion vectors.
 7. The method of claim 3, wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, the second set of motion vectors is between the second field of the first frame and the second field of the second frame; said method further comprising determining a third set of motion vectors between the first field of the first frame and the first field of the third frame; and wherein identifying further comprises comparing a first ratio of the first set of motion vectors and the third set of motion vectors to the first threshold, comparing a second ratio of the second set of motion vectors and the first set of motion vectors to a second threshold, determining whether a repeated field exists if the first ratio is less than the first threshold and the second ratio is greater than the second threshold.
 8. The method of claim 1, wherein the first threshold is a heuristically determined value.
 9. The method of claim 1, wherein if a repeated field is identified, replacing the repeated field with a reference to a field from which the repeated field is repeated.
 10. The method of claim 1, wherein if a repeated field is identified, averaging the repeated field and a field from which the repeated field is repeated.
 11. A video device comprising: an input configured to received input video frames; motion vector logic determining a first set and second set of field motion vectors; a processor identifying a repeated field by comparing a ratio of the first and second sets of field motion vectors to a threshold.
 12. The video device of claim 11 wherein if the processor identifies a repeated field, the processor to replace the repeated field with a reference to a field from which the repeated field is repeated.
 13. The video device of claim 11, wherein if the processor identifies a repeated field, the processor to average the repeated field with a reference field from which the repeated field is repeated.
 14. The video device of claim 11 wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame; said logic further determining a third set of motion vectors between the first field of the second frame and the first field of the third frame; and wherein identifying further comprises comparing a first ratio of the first set of motion vectors and the second set of motion vectors to the first threshold, comparing a second ratio of the first set of motion vectors and the third set of motion vectors to the first threshold, and identifying a repeated field if the first ratio and the second ratio are less than the first threshold.
 15. The video device of claim 11 wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, the second set of motion vectors is between the second field of the first frame and the second field of the second frame; said logic a third set of motion vectors between the first field of the first frame and the first field of the third frame; and wherein identifying further comprises comparing a first ratio of the first set of motion vectors and the third set of motion vectors to the first threshold, comparing a second ratio of the second set of motion vectors and the first set of motion vectors to a second threshold, and identifying a repeated field if the first ratio is less than the first threshold and the second ratio is greater than the second threshold.
 16. The video device of claim 11 wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, the second set of motion vectors is between the second field of the first frame and the second field of the second frame; said logic further determining a third set of motion vectors between the first field of the first frame and the first field of the third frame and a fourth set of motion vectors between the second field of the first frame and the second field of the third frame; and wherein identifying further comprises comparing a first ratio of the sum of the first set of motion vectors minus the third set of motion vectors and the first set of motion vectors to the first threshold, comparing a second ratio of the sum of the second set of motion vectors minus the fourth set of motion vectors and the sum of the first set of motion vectors minus the third set of motion vectors to a second threshold, comparing the third set of motion vectors to the fourth set of motion vectors, and identifying a repeated field if the first ratio is less than the first threshold, the second ratio is greater than the second threshold, and the magnitude of the third set of motion vectors is less than the magnitude of the fourth set of motion vectors.
 17. The video device of claim 11, wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, the second set of motion vectors is between the second field of the first frame and the second field of the second frame; said logic further determining a third set of motion vectors between the first field of the first frame and the first field of the third frame; and wherein identifying further comprises comparing a first ratio of the first set of motion vectors and the third set of motion vectors to the first threshold, comparing a second ratio of the second set of motion vectors and the first set of motion vectors to a second threshold, determining whether a repeated field exists if the first ratio is less than the first threshold and the second ratio is greater than the second threshold.
 18. A computer readable medium storing executable computer program instructions which, when executed by a processor, cause the processor to perform a method comprising: determining a first and second set of motion vectors from input video frames; determining whether a repeated field exists by comparing the first and second set of motion vectors to a first threshold.
 19. The computer readable medium of claim 18, wherein if a repeated field is found, replacing the repeated field with a reference to a field from which the repeated field is repeated.
 20. The computer readable medium of claim 18, wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, and the second set of motion vectors is between the second field of the first frame and the second field of the second frame, the method further comprising: determining a third set of motion vectors between the first field of the second frame and the first field of the third frame; and wherein identifying further comprises comparing a first ratio of the first set of motion vectors and the second set of motion vectors to the first threshold, comparing a second ratio of the first set of motion vectors and the third set of motion vectors to the first threshold, and identifying a repeated field if the first ratio and the second ratio are less than the first threshold.
 21. The computer readable medium of claim 18, wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, the second set of motion vectors is between the second field of the first frame and the second field of the second frame; said method further comprising determining a third set of motion vectors between the first field of the first frame and the first field of the third frame; and wherein identifying further comprises comparing a first ratio of the first set of motion vectors and the third set of motion vectors to the first threshold, comparing a second ratio of the second set of motion vectors and the first set of motion vectors to a second threshold, and identifying a repeated field if the first ratio is less than the first threshold and the second ratio is greater than the second threshold.
 22. The computer readable medium of claim 18 wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, the second set of motion vectors is between the second field of the first frame and the second field of the second frame; said method further comprising determining a third set of motion vectors between the first field of the first frame and the first field of the third frame and a fourth set of motion vectors between the second field of the first frame and the second field of the third frame; and wherein identifying further comprises comparing a first ratio of the sum of the first set of motion vectors minus the third set of motion vectors and the first set of motion vectors to the first threshold, comparing a second ratio of the sum of the second set of motion vectors minus the fourth set of motion vectors and the sum of the first set of motion vectors minus the third set of motion vectors to a second threshold, comparing the third set of motion vectors to the fourth set of motion vectors, and identifying a repeated field if the first ratio is less than the first threshold, the second ratio is greater than the second threshold, and the magnitude of the third set of motion vectors is less than the magnitude of the fourth set of motion vectors.
 23. The computer readable medium of claim 18 wherein the first set of motion vectors is between the first field of the first frame and the first field of the second frame, the second set of motion vectors is between the second field of the first frame and the second field of the second frame; said method further comprising determining a third set of motion vectors between the first field of the first frame and the first field of the third frame; and wherein identifying further comprises comparing a first ratio of the first set of motion vectors and the third set of motion vectors to the first threshold, comparing a second ratio of the second set of motion vectors and the first set of motion vectors to a second threshold, determining whether a repeated field exists if the first ratio is less than the first threshold and the second ratio is greater than the second threshold. 